Hyperbolic metamaterial-based near-field transducer for heat assisted magnetic recording head

ABSTRACT

An apparatus comprises a slider configured for heat-assisted magnetic recording. A near-field transducer comprising a peg is situated at or near an air bearing surface of the slider, and an optical waveguide of the slider is configured to couple light from a light source to the near-field transducer. The peg comprises a hyperbolic metamaterial, and the near-field transducer may further include an enlarged portion from which the peg extends, where the enlarged portion may also comprise a hyperbolic metamaterial.

RELATED APPLICATIONS

This application claims the benefit of Provisional Patent ApplicationSer. No. 62/296,938 filed on Feb. 18, 2016, and Provisional PatentApplication Ser. No. 62/362,372 filed on Jul. 14, 2016, to whichpriority is claimed pursuant to 35 U.S.C. § 119(e) and which are herebyincorporated herein by reference in their entirety.

SUMMARY

Embodiments discussed herein involve an apparatus comprising a slider, anear-field transducer (NFT), and an optical waveguide. The slider isconfigured for heat-assisted magnetic recording, and the waveguide ofthe slider is configured to couple light from a light source to thenear-field transducer. The near-field transducer comprises a pegsituated at or near an air bearing surface of the slider, and the pegcomprises a hyperbolic metamaterial.

Further embodiments are directed to an apparatus comprising a sliderconfigured for heat-assisted magnetic recording, an NFT, and an opticalwaveguide. The optical waveguide is configured to couple light from alight source to the NFT. The NFT comprises a peg situated at or near anair bearing surface of the slider. The peg comprises a plurality ofsublayers, where the sublayers comprise alternating a metallic sublayerand an insulating sublayer to comprise a hyperbolic metamaterial.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures.

FIG. 1 is a perspective view of a heat assisted magnetic recording(HAMR) slider in accordance with various embodiments discussed herein;

FIG. 2 is a perspective view of a HAMR slider in accordance with variousembodiments discussed herein;

FIG. 3 is a side view of a HAMR slider in accordance with variousembodiments discussed herein;

FIGS. 4A-C are cross-sectional views of a portion of a slider configuredfor HAMR in accordance with various embodiments discussed herein;

FIG. 5A is a cross-sectional view of a single piece NFT NTS design inaccordance with various embodiments discussed herein;

FIG. 5B is a perspective view of a NFT NTL design in accordance withvarious embodiments discussed herein;

FIG. 5C illustrates a shape of an NTS NFT design in accordance withvarious embodiments discussed herein;

FIG. 5D illustrates a shape of a PPG NFT design in accordance withvarious embodiments discussed herein;

FIG. 5E illustrates a shape of a NTP NFT design in accordance withvarious embodiments discussed herein;

FIGS. 5F-H show views of an aperture NFT in accordance with variousembodiments discussed herein;

FIG. 5I illustrates a perspective view of an aperture NFT from the ABSin accordance with various embodiments discussed herein;

FIG. 5J is a close-up view of the hyperbolic metamaterial of FIG. 5I inaccordance with various embodiments discussed herein;

FIG. 5K is a field profile of a plasmon wave exciting a hyperbolicmetamaterial peg in accordance with various embodiments discussedherein;

FIG. 5L is a temperature profile of a hyperbolic metamaterial peg inaccordance with various embodiments discussed herein;

FIGS. 6A-C are cross-sectional diagrams of a NFT peg comprising ahyperbolic metamaterial in accordance with various embodiments discussedherein;

FIG. 7A shows a peg of a near-field transducer comprising a hyperbolicmetamaterial in accordance with various embodiments discussed herein;

FIG. 7B shows a profile of a hotspot produced on a magnetic recordingmedium by excitation of the highest order modes propagating within thehyperbolic metamaterial peg of FIG. 7A;

FIG. 8A shows a peg of a near-field transducer comprising a hyperbolicmetamaterial in accordance with various embodiments discussed herein;

FIG. 8B shows a profile of a hotspot produced on a magnetic recordingmedium by excitation of the highest order modes propagating within thehyperbolic metamaterial peg of FIG. 8A;

FIG. 9A shows an electric field profile of the lowest TM (transversemagnetic) mode excited in a hyperbolic metamaterial peg such as that ofFIG. 7A;

FIG. 9B shows a profile of a hotspot produced on a magnetic recordingmedium by excitation of the lowest TM mode propagating within thehyperbolic metamaterial peg of FIG. 9A;

FIG. 10A shows an electric field profile of the lowest TM mode excitedin a hyperbolic metamaterial peg such as that of FIG. 8A;

FIG. 10B shows a profile of a hotspot produced on a magnetic recordingmedium by excitation of the lowest TM mode propagating within thehyperbolic metamaterial peg of FIG. 10A;

FIG. 11A shows a hotspot produced on a magnetic recording medium using ahyperbolic metamaterial peg design in accordance with variousembodiments discussed herein; and

FIG. 11B shows a comparison of the high thermal gradient of a homogenousplasmonic material peg with that of a hyperbolic metamaterial peg inaccordance with various embodiments discussed herein.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

Heat assisted magnetic recording (HAMR), also sometimes referred to asthermal-assisted magnetic recording (TAMR) or energy assisted magneticrecording (EAMR), is a technology that enables areal storage density inhard disk drives well beyond 1 Tb/in², e.g., to 5 Tb/in² in theory,using the high magnetocrystalline anisotropy of FePt. The recordingprocess starts by heating a small region of the disk above Curietemperature (T_(c)) using a laser powered near-field plasmonictransducer. The region is subsequently cooled rapidly in the presence ofa magnetic field from the recording head. The magnetic field maintainsthe orientation of magnetization in the local region of heated media asit cools, thereby encoding a bit with data for storage. By reducing thelocal media anisotropy (K) at high temperature (e.g., above T_(c)), HAMRmakes it possible to record data on high anisotropy material such as L1₀-FePt. The high anisotropy in L1 ₀-FePt extends the superparamagneticlimit faced with conventional magnetic recording, so that grain size canbe further reduced to increase signal-to-noise ratio and area density.The heated area in the storage layer of the recording medium determinesthe data bit dimensions, and linear recording density is determined bythe sharpness of magnetic transitions between the data bits.

In order to achieve desired data density, a HAMR recording head (e.g.,slider) includes optical components that direct light from a laser tothe recording media. The HAMR media hotspot may need to be smaller thana half-wavelength of light available from current sources (e.g., laserdiodes). Due to what is known as the diffraction limit, opticalcomponents cannot focus the light at this scale. One way to achievetiny, confined hot spots is to use an optical NFT, such as a plasmonicoptical antenna. The NFT is designed to support local surface-plasmonsat a designed light wavelength. At resonance, high electric fieldsurrounds the NFT due to the collective oscillation of electrons in themetal. Part of the field will get absorbed by a storage medium, raisingthe temperature of the medium locally for recording. During recording, awrite element (e.g., write pole) applies a magnetic field to the heatedportion of the medium. The heat lowers the magnetic coercivity of themedia, allowing the applied field to change the magnetic orientation ofthe heated portion. The magnetic orientation of the heated portiondetermines whether a one or a zero is recorded. By varying the magneticfield applied to the magnetic recording medium while it is moving, datais encoded onto the medium. However, in HAMR the propensity of plasmonicNFT materials to undergo physical and/or chemical changes under largethermal stress impacts the stability and reliability of the head's lightdelivery structure. Plasmonic materials are those that have the realpart of the electric permittivity less than zero—if refractive index isn+ik where n is the real part and k is the imaginary part, plasmonicmaterials have k>n. Gold and Au-based alloys are some examples ofplasmonic NFT materials. HAMR head failure can then result from NFTpeg-enlarged portion separation, peg deformation, and/or chemicalmodification. Embodiments described herein are directed to replacing theplasmonic elemental metal or metal alloy materials of the NFT peg and/orenlarged portion with metal/ceramic nanostructured materials known ashyperbolic metamaterials.

A HAMR drive uses a laser diode to heat the media to aid in therecording process. Due to inefficiencies of electric to optical power,the laser diode also heats itself during lasing. Components (writer,reader, heat elements) in the magnetic slider also dissipate heat andthe heat is conducted to the laser diode as the laser diode submount ismounted on the slider. To illustrate possible optical transmissionpaths, FIGS. 1 and 2 show perspective views of HAMR configurationsaccording to example embodiments. In FIG. 1, slider 100 has alaser-in-slider (LIS) configuration. In this configuration, slider 100includes a slider body 101 having an edge-emitting laser diode 102integrated into a trailing edge surface 104 of the slider body 101. Inthis example, the laser diode 102 is disposed within a cavity formed inthe trailing edge surface 104. The laser diode 102 is proximate to aHAMR read/write element 106, which has one edge on an air bearingsurface 108 of the slider 100. The air bearing surface 108 faces and isheld proximate to a moving media surface (not shown) during deviceoperation.

While here the read/write element 106 is shown as a single unit, thistype of device may have a physically and electrically separate readelement (e.g., magnetoresistive stack) and write element (e.g., a writecoil and pole) that are located in the same general region of the slider100. The separate read and write portion of the read/write element 106may be separately controlled (e.g., having different signal lines,different head-to-media spacing control elements, etc.), although theymay share some common elements (e.g., common signal return path). Itwill be understood that the concepts described herein apply to devicesthat have one or multiple write elements on a given recording head.

The laser diode 102 provides electromagnetic energy to heat the mediasurface at a point near to the read/write element 106. Optical pathcomponents, such as a waveguide 110, are formed integrally within theslider 100 to deliver light from the laser diode 102 to the media. Inparticular, a local waveguide and NFT 112 may be located proximate theread/write element 106 to provide local heating of the media duringwrite operations.

In FIG. 2, a laser-on-slider (LOS) configuration 120 is illustrated.This example includes a laser diode 122 that is mounted on a top surfaceof a slider body 121. The laser diode 122 is coupled to an optical pathof the slider body 121 that includes, among other things, an opticalpath 124 (e.g., a straight waveguide). In this configuration, the laserdiode 122 may also be edge-emitting, such that the light is emitted fromthe laser diode 122. In order to direct the light towards the airbearing surface 108, the laser diode 122 (or other component) mayinclude optical path elements such as a mirror (not shown) thatredirects the light emitted from the laser diode 122 towards the airbearing surface 108. In other configurations, an edge-emitting,top-mounted laser diode may be oriented so that the light emitteddirectly downwards toward the air bearing surface 108. This may involveplacing the laser diode 122 on a submount (not shown) on the top of theslider body 121, the submount orienting the laser output in the desireddirection.

While other components shown in FIG. 2, such as the NFT 112 andread/write element 106, are referenced using the same numbers as FIG. 1,the physical configuration of these and other components may differ inthe different slider arrangements, e.g., due to the differences inoptical coupling pathways, materials, laser power, etc. While notillustrated explicitly in FIGS. 1 and 2, slider configurations mayutilize different types of semiconductor laser diodes, such as lasershaving a Fabry-Perot laser diode cavity, a distributed Bragg reflector(DBR) laser, and a distributed feedback (DFB) laser. The embodimentsdescribed below may be applicable to a variety of energy deliveryconfigurations and laser diode types.

Referring now to FIG. 3, a block diagram shows a side view of a slider302 according to a representative embodiment. The slider 302 may be usedin a magnetic data storage device, e.g., hard drive, configured forHAMR. The slider 302 may also be referred to herein as a read/writehead, recording head, etc. The slider 302 is coupled to an arm 304 byway of a suspension 306 that allows some relative motion between theslider 302 and arm 304. The slider 302 includes writing components 308at a trailing edge that are held proximate to a surface 310 of amagnetic recording medium 311, e.g., magnetic disk. The slider 302 isconfigured as a HAMR recording head, which includes a laser 320 and awaveguide 322. The waveguide 322 delivers light from the laser 320 tothe writing components 308.

When the slider 302 is located over surface 310 of recording medium 311,a flying height 312 is maintained between the slider 302 and the surface310 by a downward force of arm 304. This downward force iscounterbalanced by an air cushion that exists between the surface 310and an air bearing surface 303 (also referred to herein as a“media-facing surface”) of the slider 302 when the recording medium 311is rotating. It is desirable to maintain a predetermined slider flyingheight 312 over a range of disk rotational speeds during both readingand writing operations to ensure consistent performance. Region 314 is a“close point” of the slider 302, which is generally understood to be theclosest spacing between the writing components 308 and the magneticrecording medium 311, and generally defines the head-to-medium spacing313. To account for both static and dynamic variations that may affectslider flying height 312, the slider 302 may be configured such that aregion 314 of the slider 302 can be configurably adjusted duringoperation in order to finely adjust the head-to-medium spacing 313. Thisis shown in FIG. 3 by a dotted line that represents a change in geometryof the region 314. In this example, the geometry change may be induced,in whole or in part, by an increase or decrease in temperature of theregion 314 via a heater 316.

FIGS. 4A-C show detailed partial cross-sectional views of the HAMRslider 400 in accordance with various embodiments. Unless otherwiselabeled, corresponding components in the figures are the same. Thewaveguide 410 includes a layer of core material 430 surrounded by firstand second cladding layers 432 and 434. The first cladding layer 432 isshown proximate the NFT 412 and the write pole 426. As can be seen inFIG. 4A, the NFT 412 includes an enlarged region 413 and a peg 415extending from the enlarged region 413. In some embodiments, the NFT 412can have a nanorod design, comprising just the peg 415 (i.e., excludingthe enlarged region 413). The enlarged region 413 may intersect both thecladding layer 432 and the core layer 430 (as shown) or optionally itmay be entirely contained within the cladding layer 432 (not shown) orentirely contained within the core layer 430 (not shown). The secondcladding layer 434 is spaced away from the first cladding layer 432 andseparated therefrom by the waveguide core 430. The core layer 430 andcladding layers 432 and 434 may be fabricated from dielectric materials,such as optical grade amorphous material with low thermalconductivities. The first and second cladding layers 432 and 434 mayeach be made of the same or a different material.

The core 430 and cladding layers 432 and 434 may generally be part of alight delivery arrangement or system that receives light from a source402 (e.g., laser diode) and directs it to the NFT 412. The materials areselected so that the refractive index of the core layer 430 is higherthan refractive indices of the cladding layers 432 and 434. Thisarrangement of materials facilitates efficient propagation of lightthrough the waveguide core 430. Optical focusing elements (not shown)such as mirrors, lenses, etc., may be utilized to concentrate light ontothe NFT 412. These and other components may be built on a commonsubstrate using wafer manufacturing techniques known in the art. Thewaveguide 410 may be configured as a planar waveguide or channelwaveguide.

As illustrated, the NFT 412 is positioned proximate the air bearingsurface (ABS) 414 (also referred to as a media-facing surface) and thewrite pole 426. Preferably, a portion of the NFT 412 (e.g., the enlargedportion 413) directly contacts the write pole 426 to provide heatsinkingfor the NFT 412. Alternatively, additional heat sinking infrastructurecan be added in addition to the write pole to assist in heat sinking(not shown). The ABS 414 is shown positioned proximate the surface 416of the magnetic recording medium 418 during device operation. In theorientation illustrated in FIG. 4A, the ABS 414 is arranged parallel tothe x-z plane. Electromagnetic energy (e.g., laser light) 420 from thelight source (e.g., laser diode) 402 is delivered to the core 430 of thewaveguide 410 via the coupler 407 and propagates in the y-directiontoward the NFT 412. The optical wave 420 is delivered to the NFT 412along the waveguide 410 in the y-direction. The NFT 412 generatessurface plasmon enhanced near-field electromagnetic energy proximate thesurface 416 of the medium 418, and the energy exits the peg 415 in they-direction. This results in production of a highly localized hot spot419 (e.g., about 50 nm in width) on the media surface 416. The writepole 426 generates a magnetic field (e.g., in a y- or perpendiculardirection) used for changing the magnetic orientation of the mediawithin hot spot 419 during writing.

FIGS. 4B-C illustrate alternative NFT configurations within a slider 400cross-section. FIG. 4B illustrates an aperture NFT design where theenlarged portion 413 involves two regions flanking and/or overlappingthe waveguide core 430. Between the two enlarged portions 413 is an area440 that is an aperture that can be partially, or fully, filled with ahyperbolic metamaterial to form a peg. FIG. 4C shows an alternativeembodiment of an aperture NFT design. Instead of two enlarged portions413, the enlarged portion 413 of FIG. 4B located further from the writepole 426 is replaced with a heatsink 444, e.g., comprised of gold. Thearea 442 between the heatsink 444 and enlarged portion 413 is anaperture that can be partially, or completely, filled with a hyperbolicmetamaterial to form a peg.

Various NFT designs can include a peg in accordance with embodimentsdiscussed herein. For example, an NFT can have a single piece designcomprising a unitary body including both an enlarged portion and a pegportion or a two-piece design comprising an enlarged portion and a pegportion formed separately. A single piece design results from depositionof the same materials to form both the enlarged and peg portions of theNFT, whereas a two-piece design can involve deposition of differingmaterials. A two-piece design can also involve materials of similar oridentical composition formed in two steps. An NFT peg comprised ofhyperbolic metamaterials, as discussed herein, can also be referred toas a “meta-peg.”

FIGS. 5A-I illustrate examples of NFT designs. FIG. 5A is across-sectional view of a stadium-style (NTS) design. The illustratedportion of NFT 500 includes an enlarged region 502, a peg region 504 anda heatsink portion 506. The peg 504 extends from the enlarged region 502toward the media-facing surface 108 (in the light-propagating, or y,direction). The peg region 504 terminates at a distal end at orproximate the air bearing surface 108. The NFT 500 design is shown toinclude a tapered portion 503 facing away from the heatsink portion 506and the write pole, not shown, and that reduces the peg dimension in thedown-track direction. FIG. 5B illustrates a lollipop-style (NTL) NFT520. Here, the peg 524 extends from an enlarged portion shaped like adisk 522. Similar to the NTS design 500, the peg 524 extends from theenlarged, disk region 522 toward the media-facing surface 108 (in thelight-propagating, or y, direction) and terminates at a distal end at orproximate the air bearing surface 108. NFT 520 is also shown with atapered peg, however, the peg need not necessarily include a taper. Asmentioned above, an NFT can also have a nanorod design, comprising justa peg and no enlarged region.

FIGS. 5C-E illustrate NFTs with approximately the same geometry as thoseof FIGS. 5A-B but with differently shaped enlarged regions. Similar toFIGS. 5A-B, the pegs 534, 544, and 554 extend from an enlarged region532, 542, and 552, respectively, toward the media-facing surface 108 (inthe light-propagating, or y, direction) and terminate at a distal end ator proximate the air bearing surface 108. The k vector represents thelight propagation direction (the y direction), and the NFT designs ofFIGS. 5A-C and 5E are oriented with the E field pointing in thetransverse (cross-track, x direction). FIG. 5D, which illustrates aplanar plasmon generator (PPG) NFT design, is oriented with the E fieldpointing in the down-track (z direction) for transverse magnetic mode.Similarly, FIG. 5E, which illustrates a NFT postage stamp (NTP) designcan be oriented with the E field in either the cross-track or down-trackdirection, and the aspect ratio of the enlarged portion 552 is adjustedto accommodate the chosen E field direction.

In reference now to FIG. 5F, an ABS view of an NFT geometry is shown inaccordance with embodiments described herein. The NFT is disposed at amedia-facing surface 108 according to an example embodiment. The x-axisin this figure is aligned in a cross-track direction, and the z-axis isaligned in a down-track direction. The aperture 572 is shown proximatean extension of the write pole 576. The plasmonic material portion 574forms a top wall that separates the aperture 572 from the write pole576. Side portions 575, 577 of plasmonic material form side wallssurround the aperture 572 in the cross-track direction. Plasmonicportion 578 forms a bottom wall of the aperture 572. These walls aregenerally normal to the media-facing surface, and surface plasmonresonance causes surface plasmons to be directed in this normaldirection to a recording media. FIG. 5G shows a top-down view of thedevice described in FIG. 5F, and FIG. 5H illustrates a cross-sectionview. The NFT shown in FIGS. 5F-H will be referred to herein as an“aperture” NFT because the ABS is mostly made of metal with an aperture572 in it. The aperture 572 allows light to travel from the waveguidecore to the recording medium. According to various implementations, anaperture NFT has one, or more, protrusion 579 protruding into theaperture 752 in the x-z plane. The protrusion 579 can comprisemultilayer hyperbolic metamaterials as described further below and beconsidered “a peg” as pegs are described throughout this disclosure.

FIG. 5I illustrates an NFT design similar to that of FIG. 5F. However,as is seen in the ABS view of FIG. 5I, the protrusion 579 is removed.The top wall 574 extends further toward, and optionally up to, the ABS,and the side portions 575, 577 are “thin” (e.g., less than 150 nmthick). Also, the aperture 582 is filled, at least partially, with ahyperbolic metamaterial 586, as described further below, to aidconfinement of the light in forming the hot spot. The hyperbolicmetamaterial forms a peg-type shape and can be considered “a peg” aspegs are described throughout this disclosure. There is no differencebetween “peg” NFT designs and “meta-peg” NFT designs whereby an apertureis at least partially filled with a hyperbolic metamaterial as bothdesigns can include peg-like structures comprising a hyperbolicmetamaterial. However, the different designs may involve different modeorders. Since FIG. 5I is a perspective view from the ABS, the dashed box588 represents the location of the waveguide behind the ABS.

FIG. 5J is a close-up view of the hyperbolic metamaterial forming thepeg of FIG. 5I. In certain embodiments, the plasmonic portion of thehyperbolic metamaterial is the same as the surrounding plasmonicmaterials for the top, bottom, and/or side walls 574, 572, 575, 577, andthe dielectric portion of the hyperbolic metamaterial is the samematerial as forms the waveguide core 588. As shown, the waveguide corematerial can extend into the aperture to form alternating layers withthe plasmonic material of the aperture NFT design's walls. However, thehyperbolic metamaterial can comprise any combination of materialsdescribed herein. FIG. 5K shows the field profile of a plasmon waveexciting the hyperbolic metamaterial peg of FIG. 5I. FIG. 5L furthershows the temperature profile of the hyperbolic metamaterial peg. Thehigh temperature areas correspond to the middle of the hyperbolicmetamaterial peg where there is high dissipation of optical energy.

The reliability of HAMR heads is an impediment against thecommercialization of HAMR drives. In particular, the lifetime andstability of the optical element of the write assembly (NFT peg andenlarged portion) are affected by the poor thermo-mechanical stabilityof the plasmonic, homogeneous metal materials (e.g., Au and Au alloys).Temperatures in a slider during writing conditions can reach above 300°C.

In conventional HAMR heads, the peg and enlarged portion are made ofhomogeneous plasmonic metals (either elemental or alloys). Suchmaterials are difficult to deposit in high quality thin film structuresdue to their high surface free energy, which leads to high filmporosity, roughness, and adhesion issues. The heat generated during theHAMR writing process has been shown to result in peg separation from theenlarged portion, deformation through densification, diffusion, graingrowth, etc., and to loss of writing ability.

Embodiments discussed herein are directed to an NFT at least partiallycomprised of a thermally stable nanostructured metal/ceramic compositematerial. The material exhibits high thermal stability and an extremelyhigh broad-band photonic density of states at the interface between themetallic and ceramic components of the composite material. Thesecomposite materials are known as hyperbolic metamaterials. Hyperbolicmetamaterials behave as a metal when light passes through them in onedirection and like a dielectric when light passes through in theperpendicular direction— also called extreme anisotropy. The namereflects the fact that the composite material's dispersion relationforms a hyperboloid. Hyperbolic metamaterials can be incorporated intoan NFT and designed to support hyperbolic dispersion of light, allowingfor the propagation of highly confined modes. This confinement not onlyallows the NFT to achieve a high thermal gradient hotspot on a magneticrecording medium but also to produce a hotspot having a predefined shapethrough the design of the hyperbolic metamaterial properties. Suchshapes can be used to optimize particular aspects of the written bits,for example, to reduce the curvature of the transitions between bits.

In some embodiments, the NFT, e.g., the peg portion, comprises anoptical hyperbolic metamaterial such as TiN/Al_(x)Sc_(1-x)N orZrN/Al_(x)Sc_(1-x)N. A nitride-based composite material exhibits a muchlower surface free energy and an enhanced surface plasmonic effect,similar to that of gold. However, other hyperbolic metamaterials wherethe dielectric portion has n>k and the metal portion has k>n, where n isthe real part of the refractive index and k is the imaginary part of therefractive index, may be used. The material properties may be furthertuned to exhibit a particular mode shape. Examples include hyperbolicmetamaterials that have at least one material resistant to chemicalchanges and/or resistant to deformation, etc. For example, Au andTaO_(x), where TaO_(x) is a robust material, Rh and SiO_(x), where Rh isa robust material, Ir and AlO_(x) where Ir is a robust material, TaO_(x)and Ir where both are robust, and TaO_(x) and TaN where both are robust.The use of O_(x) throughout this discussion, such as in TaO_(x),indicates a stoichiometric oxide of tantalum (or other compound whenused with another element such as silicon or aluminum). For example,TaO_(x) can represent TaO₂ as well as Ta₂O₅.

Thus, example combinations of materials forming hyperbolic metamaterialsapplicable to the discussions herein include Rh with SiO_(x), AlO_(x)and/or TaO_(x); Ir with SiO_(x), AlO_(x), and/or TaO_(x); and Au withSiO_(x), AlO_(x), and/or TaO_(x). Other example combinations include Cuand/or Ag with SiO_(x), AlO_(x), and/or TaO_(x); gold alloys withoxides; metal nitrides with metal nitrides such as TiN with AlN whereTiN is the metal (k>n) and AlN is a dielectric (n>k); and metal nitrideswith metal oxides such as TaN, ZrN and/or TiN with SiO_(x), AlO_(x),TiO_(x), NbO_(x), and/or TaO_(x). Further examples include Cu, Ag, Au,Rh, Ir, Pd, Pt, Os, Nb, and/or Mo with SiO_(x), AlO_(x), TiO_(x),NbO_(x), TaO_(x), amorphous Si, hydrogenated amorphous Si, SiC, and/orhydrogenated SiC. While the entire NFT may be comprised of thesecomposite materials, embodiments described herein address at least a pegstructure being formed of hyperbolic metamaterials.

In contrast to homogenous, or elemental, plasmonic materials currentlyused in the light delivery structures of HAMR heads, hyperbolicmetamaterials such as nitride-based composite materials such asTiN/Al_(x)Sc_(1-x)N and ZrN/Al_(x)Sc_(1-x)N exhibit very low surfacefree energies (e.g., ˜1,000 mJ/m² for gold vs.˜50 mJ/m² for nitrides),which allows these materials to build high quality epitaxialnanostructures (e.g., multilayers and superlattices)—even with knowndeposition techniques. Such deposition techniques can include physicalor chemical deposition methods such as radio frequency (rf) sputtering,direct current (dc) sputtering, reactive magnetron sputtering, chemicalvapor deposition, pulsed laser deposition, and molecular beam epitaxy.The use of nanostructured plasmonic composite materials in the NFTpromote optical component stability and provide long term writerstability and reliability.

An NFT peg, and/or other portions of an NFT, comprised of hyperbolicmetamaterials has an alternating thin film structure, e.g., a pluralityof bi-layers, as shown in FIG. 6. The NFT peg 600 comprises at least oneplasmonic metallic material layer 602 and at least one dielectric, orinsulating, material layer 604, i.e., at least one bi-layer structure.However, the peg 600 typically comprises a plurality of bi-layerstructures, for example, four bi-layer structures are shown. In certainembodiments, the metallic material is a nitride-based plasmonic materialsuch as TiN or ZrN. The insulating material, or ceramic dielectric,forms sharp interfaces with the plasmonic material and is also nitridebased such as Al_(x)Sc_(1-x)N. Together the hyperbolic metamaterial peg600 comprises TiN/Al_(x)Sc_(1-x)N and/or ZrN/Al_(x)Sc_(1-x)N. The amountof Sc is about 10-30% to stabilize the AlN in cubic phase such that x isabout 0.7-0.9 and in certain embodiments, x=0.88, in the formula,Al_(x)Sc_(1-x)N. For example, Al_(x)Sc_(1-x)N thin films are sandwichedbetween plasmonic layers, such as TiN, as described in U.S. PatentPublication No. 2015/0285953, incorporated herein in its entirety.Alternatively, Al_(x)Sc_(1-x)N thin films are sandwiched betweenplasmonic layers, such as ZrN, or sandwiched between alternating layersof TiN and ZrN (e.g., . . . /TiN/Al_(x)Sc_(1-x)N/ZrN/Al_(x)Sc_(1-x)N/ .. . ). Thus, the layered hyperbolic metamaterial forms at least the pegof an NFT.

The plasmonic effect of the NFT can be modified by varying aspects ofthe hyperbolic metamaterials used to form the peg and/or enlarged regionof the NFT. Both light delivery in the head as well as the media thermaldesign influence control of the hot spot. For a given media thermaldesign, the hot spot size and shape depend on many factors including theorientation of the superlattice plane of the hyperbolic metamaterialwith respect to the down-track direction, the refractive index contrastof the hyperbolic metamaterial components, the thickness of the layersforming the superlattice, laser modes, and the peg's shape and size. Forexample, the plasmonic effect of the peg can be enhanced by changing,such as reducing, the thickness of one or both of the metallic anddielectric layers as well as altering the number of layers. Also, thehyperbolic metamaterial stoichiometry can be altered based upon thewavelengths emitted by the laser to minimize losses and improve couplingefficiency between the head and recording media. For example, thestoichiometry of the TiN, or plasmonic material component, of thehyperbolic metamaterial peg can be altered to enhance the plasmoniceffect of the NFT within a desired wavelength range. In otherembodiments, the hyperbolic metamaterial can be composed of an array ofapertures and/or rods instead of layers, which are oriented to achievedesired peg properties.

Further, the orientation of the hyperbolic metamaterial superlatticestructures can be modified to achieve a desired light/hot spot geometryand to enhance the plasmonic effect in a desired direction. For example,the orientation of the superlattice plane of the hyperbolic metamaterialwith respect to the down-track direction can be used to either enhancethe down-track thermal gradient or to reduce the curvature of thetransitions. The down-track or cross-track profile of the peg can beadjusted to modify the written track width and the down-track gradient.In certain embodiments, a narrow peg in the cross-track direction writesa narrow track, and in most embodiments, reducing the down-trackthickness of the peg will increase the down-track thermal gradient andtherefore the areal density.

Using FIGS. 4A-6C as an example, embodiments are directed to anapparatus comprising a slider 400 configured for HAMR. A near-fieldtransducer 412 is situated at or near an air bearing surface 414 of theslider 400. An optical waveguide of the slider 410 is configured tocouple light from a light source 402 to the NFT 412. The NFT 412includes a peg 415, which comprises a hyperbolic metamaterial. In someembodiments, the NFT 412 includes a peg 415 and an enlarged portion 413,both of which comprise a hyperbolic metamaterial. The hyperbolicmetamaterial forms the peg 600 in a multilayered structure, as shown inFIG. 6. The alternating layers of material are orientated along the airbearing surface 414 such that each layer 602 and 604 is proximate theair bearing surface 414 (e.g., the cross-section of FIG. 6 faces therecording medium). Alternatively, this can be illustrated as shown inFIG. 6B where the layers 612, 614 of peg 610 extend vertically from theABS into the slider such as when the layers 612, 614 are deposited on asidewall. In further embodiments, hyperbolic metamaterial layers 622,624 can be deposited in a spacer-like process on a step feature to formthe peg 620 of FIG. 6C.

Turning now to FIG. 7A, there is shown a cross-sectional view of an NFTpeg 700 comprising a hyperbolic metamaterial in accordance with variousembodiments. The hyperbolic metamaterial is structured as a stack ofmultilayers, such as ten layers comprising five bi-layer structures. Asdiscussed above, the bi-layer structures are a combination of a metallayer 702, here gold (Au), and a dielectric layer 704, here tantalumoxide (TaO_(x)). The peg 700 dimensions were arbitrarily selected toprovide a cross-sectional area of 50 nm by 50 nm, and each of the layers702 and 704 is five nanometers thick, for non-limiting illustrativepurposes. The cross-sectional area shown in FIG. 7A is planar with, orparallel to, the air-bearing surface of a HAMR slider. For example, theterminal end of the NFT peg having the cross-sectional area shown may beexposed at the air-bearing surface, or may be recessed within therecording head and parallel to the air-bearing surface. Further detailsof the NFT peg having the cross-sectional area shown in FIG. 7A includeusing gold as a heatsink. In certain embodiments, the cross-sectionalarea of FIG. 7A can represent a peg as shown in the NFT design of FIG.5J, where the NFT gap can be formed of aluminum oxide (e.g., low indexmaterial) and the exciting waveguide can be tantalum oxide. As analternative to the materials discussed above, these embodiments involvea substantial index contrast in the materials forming the hyperbolicmetamaterial. For example, one material has a high index and the other alow index where n1/n2>1.5 and k1˜k2˜0. Some example materials for n1include high index materials such as silicon, silicon carbide, aluminumnitride, silicon nitride, SiO_(x), AlO_(x), HfO_(x), SrTiO_(x), TaO_(x),BaTiO_(x), NbO_(x), BiTiO_(x), TiO_(x), diamond, etc. For n2, someexample low index materials include MgF, MgO, SiO_(x), AlO_(x), TaO_(x),etc.

FIG. 7B shows a profile of a hotspot 710 produced on the surface of amagnetic recording medium using the hyperbolic metamaterial peg 700shown in FIG. 7A. The hotspot profile 710 corresponds to an excitationwith the highest order modes supported by the hyperbolic metamaterial ofpeg 700. To illustrate the shape of the hotspot 710, FIG. 7B includes anoutline of the hyperbolic metamaterial peg 700 overlaid upon theresulting hotspot 710. For a Au/TaO_(x) peg design, the resultinghotspot is elongated along the middle of the peg 700 in the down-trackdirection to achieve increased track density. However, the hot spot canalso or optionally be elongated in the cross-track direction by tuningdeposition of the layers forming the hyperbolic metamaterial peg 700.For example, the hot spot can be elongated by increasing the width ofthe hyperbolic metamaterial.

Similar to FIG. 7A, FIG. 8A shows a cross-sectional view of an NFT peg800 comprising a hyperbolic metamaterial in accordance with variousembodiments. The hyperbolic metamaterial is structured as a stack ofmultilayers, such as ten layers comprising five bi-layer structures. Asdiscussed above, the bi-layer structures are a combination of a metallayer 802, here rhodium (Rh), and a dielectric layer 804, here tantalumoxide (TaO_(x)). The peg 800 dimensions were arbitrarily selected toprovide a cross-sectional area of 50 nm by 50 nm, and each of the layers802 and 804 is five nanometers thick, for non-limiting illustrativepurposes. The cross-sectional area shown in FIG. 8A is planar with, orparallel to, the air-bearing surface of a HAMR slider. For example, theterminal end of the NFT peg having the cross-sectional area shown may beexposed at the air-bearing surface, or may be recessed within therecording head and parallel to the air-bearing surface. Further detailsof the NFT peg having the cross-sectional area shown in FIG. 8A includeusing gold as a heatsink. In certain embodiments, the cross-sectionalarea of FIG. 8A can represent a peg as shown in the NFT design of FIG.5J, where the NFT gap can be formed of aluminum oxide (e.g., low indexmaterial) and the exciting waveguide can be tantalum oxide or othermaterials having a substantial index contrast.

FIG. 8B shows a profile of a hotspot 810 produced on the surface of amagnetic recording medium using the hyperbolic metamaterial peg 800shown in FIG. 8A. The hotspot profile 810 corresponds to an excitationwith the highest order modes supported by the hyperbolic metamaterial ofpeg 800. To illustrate the shape of the hotspot 810, FIG. 8B includes anoutline of the layers of the hyperbolic metamaterial peg 800 overlaidupon the resulting hotspot 810. For a Rh/TaO_(x) peg design, theresulting hotspot is larger and takes on a squared shape largelycorresponding to the cross-sectional area of the hyperbolic metamaterialpeg 800. Additionally, the Rh/TaO_(x) design has lower curvature thanthe Au/TaO_(x) design as illustrated from both FIGS. 8A-B.

The hyperbolic metamaterial NFT peg increases both cross-track anddown-track thermal gradients as compared with a peg comprised ofhomogenous plasmonic material. The down-track thermal gradient (TG) forpeg 700 of FIGS. 7A-B (Au/TaO_(x) layers) is 20.24 K/nm(Kelvin/nanometer) and the cross-track thermal gradient is 10.17 K/nm.The down-track thermal gradient for peg 800 of FIGS. 8A-B (Rh/TaO_(x)layers) is 19.63 K/nm and the cross-track thermal gradient is 20.79K/nm. These values can be compared with a non-metamaterial peg, forexample, where a gold (Au) peg has a down-track thermal gradient of 5.28K/nm and a cross-track thermal gradient of 5.05 K/nm. These gradientsare obtained at a contour of T=405° C. to achieve a track width of 50nm. These high order modes are excited through transverse electric (TM)polarization—electric field polarized perpendicular to the multilayerplane. Lower order modes also offer an increase in the thermal gradientof hyperbolic metamaterial pegs as compared with non-metamaterial pegs,which is described below with reference to FIGS. 9A-10B.

FIG. 9A shows the electric field profile of the lowest TM mode excitedin a hyperbolic metamaterial peg 900. The peg 900 comprises a hyperbolicmetamaterial comprising a plurality of bi-layer structures where theplasmonic layer 902 is Au and the dielectric layer 904 is TaO_(x).Similar to the peg 700 in FIG. 7A, there are five bi-layer structuresfor a total of ten layers. FIG. 10A shows the electric field profile ofthe lowest TM mode excited in a hyperbolic metamaterial peg 1000. Thepeg 1000 comprises a hyperbolic metamaterial comprising a plurality ofbi-layer structures where the plasmonic layer 1002 is Rh and thedielectric layer 1004 is TaO_(x). Similar to the peg 800 in FIG. 8A, peg1000 has five bi-layer structures for a total of ten layers. It is notedthat these lowest modes can be excited with a relatively simple NFTdesign, such as a plasmonic generator (PPG design), NTS design, NTLdesign, or an aperture design. FIGS. 9B and 10B show profiles ofhotspots 910 and 1010 obtained for the modes excited in the hyperbolicmetamaterial pegs 900 and 1000, respectively.

Similar to the high order modes, the low order modes also increase bothcross-track and down-track thermal gradients in hyperbolic metamaterialNFT pegs as compared with a peg comprised of homogenous plasmonicmaterial. The down-track thermal gradient (TG) for peg 900 of FIGS. 9A-B(Au/TaO_(x) layers) is 13.10 K/nm and the cross-track thermal gradientis 8.87 K/nm. The down-track thermal gradient for peg 1000 of FIGS.10A-B (Rh/TaO_(x) layers) is 10.97 K/nm and the cross-track thermalgradient is 14.16 K/nm. These values can be compared with anon-metamaterial peg where a gold (Au) peg has a down-track thermalgradient of 6 K/nm and a cross-track thermal gradient of 6 K/nm.

FIGS. 11A-B illustrate the differences between an excitation with a pegof homogenous plasmonic material and a hyperbolic metamaterial peg. Forexample, FIG. 11A is a profile of a hotspot 1110 produced on the surfaceof a magnetic recording medium using a Rh/TaO_(x) peg 1100. The peg 1100was analyzed for the highest order mode within the hyperbolicmetamaterial, normalizing the hotspot 1110 to a maximum temperature ofthe media of 500° C., which takes into account possible damage producedto lube around the recording layer. Comparison of the down-track thermalgradient (TGDT) as a function of track width (TW in nm) for differentNFT designs indicates that the thermal gradient obtained by theRh/TaO_(x) hyperbolic metamaterial peg 1100 at a track width of 46 nm is14.25 K/nm down-track and 10.96 K/nm cross-track. At a track width of 46nm, the thermal gradient for the Rh/TaO_(x) hyperbolic metamaterial peg1100 was the highest of the compared designs.

Further, FIG. 11B shows that due to the interference of multiple spotsit is possible to obtain a high thermal gradient 1124 for a high trackwidth, thus achieving the squaring of the hotspot 1110. FIG. 11B furthershows the difference between a high thermal gradient 1122 obtained via anarrow, homogenous material peg and a high thermal gradient 1124obtained through the Rh/TaO_(x) hyperbolic metamaterial peg 1100. It canbe seen that a substantially square thermal gradient 1124 can beobtained using the hyperbolic metamaterial peg 1100. This square shapedhotspot 1110 and thermal gradient 1124 provide higher bit density, canbetter cover a larger track width, and help reduce the amount of headenergy dissipated as power distribution is larger than for a narrowerpeg design (e.g., a peg of homogenous plasmonic material).

Alternatively, the hyperbolic metamaterials described herein asmultilayer structures can be fabricated as a granular two-phasecomposition. The hyperbolic metamaterial NFT peg and/or enlarged portionstill comprises a metallic material and an insulating material. Theinsulating material, or ceramic dielectric, forms sharp interfaces withthe plasmonic, metallic material. In certain embodiments, the hyperbolicmetamaterial comprises TiN/Al_(x)Sc_(1-x)N, ZrN/Al_(x)Sc_(1-x)N,Au/TaO_(x), and/or Rh/TaO_(x). The stoichiometry can be altered basedupon the wavelengths emitted by the laser to minimize losses and improvecoupling efficiency with the recording medium. In the two-phasestructure of the hyperbolic metamaterial one material comprises “grains”and the other a “segregant” or matrix material surrounding the grains.In certain embodiments the plasmonic, metallic material can form thegrains while the dielectric material forms the segregant, and in otherembodiments, the dielectric material forms the grains with the plasmonicmaterial forming the segregant. The grains can be about 5-10 nm orsmaller in diameter such that the NFT peg can also be referred to as ananocomposite layer. The plasmonic effect of the hyperbolic metamaterialcomposite layer can be altered by increasing the volume of the metallicmaterial (e.g., TiN, ZrN, Au, and/or Rh) in the hyperbolic metamaterialsince that will increase the interface surface area with the insulatormaterial, and the sharp interface between the materials enhances theplasmonic effect. Thus, the two-phase hyperbolic metamaterial forms agranular, or rod-like structure as the NFT peg and/or enlarged portion.

Systems, devices, or methods disclosed herein may include one or more ofthe features structures, methods, or combination thereof describedherein. For example, a device or method may be implemented to includeone or more of the features and/or processes above. It is intended thatsuch device or method need not include all of the features and/orprocesses described herein, but may be implemented to include selectedfeatures and/or processes that provide useful structures and/orfunctionality.

Various modifications and additions can be made to the disclosedembodiments discussed above. Accordingly, the scope of the presentdisclosure should not be limited by the particular embodiments describedabove, but should be defined only by the claims set forth below andequivalents thereof.

What is claimed is:
 1. An apparatus, comprising: a slider configured forheat-assisted magnetic recording; a near-field transducer comprising apeg situated at or near an air bearing surface of the slider, the pegcomprising a hyperbolic metamaterial comprising TiN/Al_(x)Sc_(1-x)N; andan optical waveguide of the slider configured to couple light from alight source to the near-field transducer.
 2. The apparatus of claim 1,wherein the near-field transducer further comprises an enlarged portionfrom which the peg extends and the enlarged portion comprises ahyperbolic metamaterial.
 3. The apparatus of claim 2, wherein thehyperbolic metamaterial of the peg and of the enlarged portion is thesame.
 4. The apparatus of claim 1, wherein x is between about 0.7 andabout 0.9.
 5. An apparatus, comprising: a slider configured forheat-assisted magnetic recording; a near-field transducer comprising apeg situated at or near an air bearing surface of the slider, the pegcomprising a hyperbolic metamaterial comprising ZrN/Al_(x)Sc_(1-x)N; andan optical waveguide of the slider configured to couple light from alight source to the near-field transducer.
 6. The apparatus of claim 1,wherein the peg comprises a plurality of sublayers, the sublayerscomprising alternating a metallic sublayer and an insulating sublayer,where each sublayer is proximate the air bearing surface.
 7. Theapparatus of claim 1, wherein the near-field transducer comprises anenlarged body and the peg extends from the enlarged body.
 8. Theapparatus of claim 1, wherein the hyperbolic metamaterial is configuredto form a hotspot on a magnetic recording medium, the hotspot having apredefined shape.
 9. The apparatus of claim 1, wherein the hyperbolicmetamaterial is configured to form a hotspot on a magnetic recordingmedium, the hotspot having a substantially square shape.
 10. Anapparatus, comprising: a slider configured for heat-assisted magneticrecording; a near-field transducer comprising a peg situated at or nearan air bearing surface of the slider, the peg comprising a plurality ofsublayers, the sublayers comprising alternating a metallic sublayer andan Al_(x)Sc_(1-x)N sublayer to comprise a hyperbolic metamaterial; andan optical waveguide of the slider configured to couple light from alight source to the near-field transducer.
 11. The apparatus of claim10, wherein the metallic sublayer comprises TiN.
 12. The apparatus ofclaim 10, wherein the metallic sublayer comprises ZrN.
 13. The apparatusof claim 10, wherein the near-field transducer further comprises anenlarged portion from which the peg extends and the enlarged portioncomprises a hyperbolic metamaterial.
 14. The apparatus of claim 10,wherein the hyperbolic metamaterial is configured to form a hotspot on amagnetic recording medium, the hotspot having a substantially squareshape.
 15. The apparatus of claim 5, wherein the near-field transducerfurther comprises an enlarged portion from which the peg extends and theenlarged portion comprises a hyperbolic metamaterial.
 16. The apparatusof claim 15, wherein the hyperbolic metamaterial of the peg and of theenlarged portion is the same.
 17. The apparatus of claim 5, wherein x isbetween about 0.7 and about 0.9.
 18. The apparatus of claim 5, whereinthe peg comprises a plurality of sublayers, the sublayers comprisingalternating a metallic sublayer and an insulating sublayer, where eachsublayer is proximate the air bearing surface.
 19. The apparatus ofclaim 13, wherein the hyperbolic metamaterial of the peg and of theenlarged portion is the same.
 20. The apparatus of claim 10, wherein xis between about 0.7 and about 0.9.